[GHC] #10351: Inferred type is rejected

[GHC]

#10351: Inferred type is rejected
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"b83160d07e626bee685f329a9a73e90a4a6074ae/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b83160d07e626bee685f329a9a73e90a4a6074ae"
 Tidy up treatment of FlexibleContexts

 Previously (Trac #10351) we could get

     Non type-variable argument in the constraint: C [t]
     (Use FlexibleContexts to permit this)
     When checking that `f' has the inferred type
       f :: forall t. C [t] => t -> ()

 which is a bit stupid: we have *inferred* a type that we
 immediately *reject*.  This patch arranges that that the
 generalisation mechanism (TcSimplify.decideQuantification)
 doesn't pick a predicate that will be rejected by the
 subsequent validity check.

 This forced some minor refactoring, as usual.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10351#comment:1>
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